Another convenient parametrization of the second-order symmetric FIR
case is when the dc normalization is relaxed so that two degrees of
freedom are retained. It is then convenient to control them
as brightness
and sustain
according to the
formulas

(10.1)

(10.2)

(10.3)

where
is the period in seconds (total loop delay),
is the
desired sustain time in seconds, and
is the brightness parameter
in the interval
. The sustain parameter
is defined here as
the time to decay by dB (or
time-constants) when
brightness
is maximum (
) in which case the loop gain is
at all frequencies, or
. As the brightness is
lowered, the dc gain remains fixed at
while higher frequencies
decay faster. At the minimum brightness, the gain at half the
sampling rate reaches zero, and the loop-filter amplitude-response
assumes the form